The long range objectives of this research are to establish the role of mispairing in mutagenesis and to determine the mechanism for the environmental enhancement of substitution mutations. The importance of this project stems from the role of mutations in gene variation, both normal and pathogenic, and from a recognition that many mutagenic carcinogenic substances are a by-product of our growing industrial technology. The use of in vitro replication studies utilizing a reconstituted replication apparatus offers distinct advantages for this project. The well defined proteins and template-primers allow careful dissection of the chemistry involved in mutagenesis and the effect on this chemistry of environmental agents. The following investigations are proposed: 1) Determination of the role of the proteins of the replication apparatus on the fidelity of replication. 2) Determination of which partiular chemical modifications of a base induce a mutation. 3) Determination of the role of minor tautomeric forms and syn isomers of the nucleic acid residues in mutagenesis.